As a method replacing a conventional radiography using a combination of a radiographic film and radiographic intensifying screens, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has bee practically employed. This method employs a radiation image storage panel comprising a stimulable phosphor layer (i.e., stimulable phosphor sheet) provided on a support, and the procedure of the method comprises the steps of causing the stimulable phosphor in the phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with stimulating rays to release the radiation energy stored in the phosphor sheet as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The stimulable phosphore sheet thus processed is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
Generally, a substrate film and a protective cover film are provided on the top and bottom surfaces of the stimulable phosphor sheet, respectively. The stimulable phosphor sheet generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor with no binder. The phosphor sheet containing no binder can be formed by deposition process or sintering process. Further, the sheet comprising agglomerated phosphor soaked with a polymer is also known. In the aforementioned method, any types of the stimulable phosphor sheets are employable.
The radiation image recorded in the stimulable phosphor sheet is generally read by the steps of applying stimulating rays onto the front surface side (phosphor layer side) of the phosphor sheet, collecting light emitted by the phosphor particles by Weans of a light-collecting means from the same side, and photoelectrically converting the light into image signals. A system for reading the image from one side of the panel in this manner is referred to as “single-side reading system”. However, there is a case that the light emitted by the phosphor particles should be collected from both sides (i.e., front and the back surface sides) of the phosphor sheet. For instance, there is a case that the emitted light is desired to be collected as much as possible. There also is a case that the radiation image recorded in the phosphor layer varies along the depth of the layer, and it is desired to detect the variation. A system for reading the image from both sides of the phosphor sheet is referred to as “double-side reading system”.
The radiation image recording and reproducing method is often used in radiography for medical diagnosis. In that case, it is especially desired to reproduce a radiation image of high quality (particularly, high sharpness for high resolution) by applying a small dose of radiation. Thus, the stimulable phosphor sheet is required to have a high sensitivity and to provide an image of high quality.
In radiography for medical diagnosis, the use of a stimulable phosphor sheet of high sensitivity can reduce a dose of radiation to be applied to a patient Therefore, it is required to provide a stimulable phosphor sheet giving an image of high sharpness with high sensitivity.
Japanese Patent Provisional Publication No. 62-47600 describes a method for preparing a stimulable phosphor sheet comprising a support and a stimulable phosphor layer deposited thereon which comprises applying an electron beam to a stimulable phosphor in a vacuum to vaporize the phosphor and depositing the vaporized phosphor or source on the support, so as to columnar crystals of the stimulable phosphor on the support. The stimulable phosphor layer thus deposited on a support has cracks between each of the columnar crystals, and therefore it gives a radiation image with high sensitivity and high sharpness. In this publication, a stimulable phosphor to be deposited (such as RbBr:Tl phosphor) is pressed by a hot press to become a shaped article such as crucible. However, there are no teaching as to the pressure and a density of the pressed article. According to technical manuals of commercially available apparatuses of deposition using electron beam such as those supplied by ANELVA Corporation, ULVAC Corporation, and JEOL Corporation, the accelerating voltage of the electron beam adopted generally is in the range of 6 kV to 10 kV, or higher than 10 kV.
Japanese Patent Publication 5-17170 teaches that a method for preparing a zinc sulfide thin film for an anti-reflection of an optical lens or electroluminescent element. According to the description, a sintered zinc sulfate having a high density and a large particle size is favorably employed for preparing the film containing neither fine particles nor pin holes.
Japanese Utility Model Publication 62-173100 indicates a structure of a stimulable phosphor sheet in which a protective film is fixed onto a whole surface the phosphor layer using an adhesive. It also describes that a spacer can be placed between the support and the protective layer so as to enclose the phosphor layer with the spacer.
It is an object of the invention to provide a stimulable phosphor sheet which is able to give a radiation image with improved image quality.
Specifically, the invention has an object to give methods favorably employable for preparing a stimulable phosphor sheet which is able to give a radiation image with improved image quality.